Electrochemical behaviour and analytical detection of ammonium ions in alkali nitrate melts

Abstract

The voltammetric behaviour of ammonium ions was investigated in the (Na, K)NO 3 equimolar melt at 518 K. The voltammograms recorded at platinum, gold and vitreous carbon rotating-disc electrodes under one atmosphere of anhydrous nitrogen are characterized by three consecutive cathodic waves. The rapid disappearance of ammonium ions from the solution can be ascribed mainly to the evaporation of the salt, which is a more rapid process than thermal decomposition under the given experimental conditions. In order to be able to neglect the effect of evaporation and decomposition, the voltammograms were recorded within a few minutes from the addition of NH 4 NO 3 to the melt. The experimental ratios between the limiting currents of the three waves recorded at a vitreous carbon rotating-disc electrode, the most suitable for analytical purposes, were found to have a constant value within the explored ammonium concentration range, i.e 8×10 −5 mol kg −1 <[NH 4 + ]<5.0×10 −3 mol kg −1 . A simple mechanistic model is proposed which permits a reasonable interpretation of the experimental results and, in particular, explains the origin of the three waves and their limiting current ratios. The diffusion coefficient of ammonium ions was found to be D =(4.0±0.4) 10 −6 cm 2 s −1 at 518 K.